Variable speed induction motor drives using static inverters are widely used. When dynamic performance is not important, the speed of the motor drive is simply adjusted by changing the frequency of the input while keeping the voltage-hertz ratio constant. There are situations, however, requiring fast response, for instance, in servo-applications. Then, feedback from speed information from the rotating shaft of the machine is usually required. Such feedback is part of a closed loop for control, or it may be part of the torque control algorithm for the determination of the slip frequency (or simply slip) of the motor excitation. In such instances, a tachometer or a high-resolution encoder is typically mounted on the motor shaft in order to provide such feedback. Often, however, the output of a tach includes ripples and a special tachometer which eliminates ripple must be used if the ripple is unacceptable. Such a tachometer is described in U.S. Pat. No. 4,520,300 "Brushless Ultra-efficient Regenerative Servomechanism."
High performance speed control, where it is uneconomical or impractical to have a shaft mounted transducer is desirable. This is the case with linear motor driven transportation systems or with steel mill drives because transducer cabling is undesirable in an environment which is inhospitable around the motor. It is also the case with retrofit applications where an existing induction motor having no shaft transducer installed needs to be speed controlled. Operating a motor without speed feedback can, however, result in motor tilling. That is, as the motor nears its stopping point and a given creeping speed is desired to be commanded for the motor, too low a creep speed is commanded, because the actual speed of the motor is not known, and the motor passes its destination and reverses its direction to go back to its correct stop position.
Speed sensorless induction motor control is known. See, for example, U.S. Pat. No. 4,009,427 by Takahashi and U.S. Pat. Nos. 4,530,376 and 4,680,526, both by Okuyama, and "Speed Compensation Motor Circuit Utilizing Real Current Component" U.S. Pat. Np. 3,619,750.
Model reference adaptive control has been described in "Adaptive Control--The Mono-Reference Approach" by Yoan D. Landau, published by Marcel Dekker, Inc., New York 1979. It is known to identify the speed of an inverter-fed induction motor by the technique of model reference adaptive control See, for example, "Speed Sensorless Vector Control of Induction Motor with Mono-Reference Adaptive System" by Shinzo Tamai, Hidehiko Sugimoto, and Masao Yano, on pages 189-195, a paper print presented at an IEEE Conference in Atlanta, Georgia on 18-23 October 1987, IA Vol. 1.
Other articles of interest are: (1) "Observers for Flux Estimation in Induction Machines" by George C. Verghese and Seth R. Sanders, IEEE trans. Industrial Electronics, Vol. 35, No. 1, for Feb. 1988, pages 85-94; (2) "Vector Control System for induction Motor using a Sepped Estimation Based on Instantaneous Slip Frequency Principles", by Hirotami Nakano, Shinichi Horie, Tsuyoshi Matsuo, and Kohji Iwata, pages 95-103, Electrical Engineering in Japan, Vol. 107, No. 4, 1987.
Another speed sensorless system is shown in "Tacho-less Vector Control Adaptive System from Motor Drive" by Schauder, U.S. Pat. No. 4,862,054, which shows a reference model based on the model equation of asynchronous motor combined with an adjustable model responsive to the direct in quadrature components of the current and to an estimated speed. An adaptive mechanism counting in a P-I amplifier expands to the direct deviation between the direct in quadrature components and generates a feedback signal representing the estimated speed. The adjustable model reacts to the estimated speed signal.
Japanese Application No. 57-71295, "Speed Control for Induction Motor", shows a speed control without a speed detector by employing a double control loop with a slip angular frequency arithmetic unit and an overshoot and prevention circuit as a feedback system for speed controlling and frequency controlling amplifiers. And, Japanese Application No. 57-142188 "Controlling Device for Commutatorless Motor", shows a controlling device for a highly efficient commutatorless motor without necessity of a speed detector by controlling the prescribed gamma constantly by using a synchronizing signal from a distributor without producing an actual speed signal. And, Japanese Application No. 1-114394, "High-performance Speed Controlling Circuit for Multiphase Induction Motor based on Detection of Only Current" shows an industrial motor control without a speed detecting device, by detecting motor current only and by computing the slip angle speed of the space vector of magnetic flux.
Still another speed sensorless drive, "Speed Control System for Elevators", U.S. Pat. No. 4,982,816 shows an elevator with an induction motor drive wherein A. the output torque is determined by direct current of an inverter, B. slip frequency is determined from the torque, C. the gap between an open-loop dictated speed pattern and the actual speed is compensated by the slip calculated during acceleration and again during constant speed movement so that the open-loop control may be improved in terms of stop position precision.